A mass spectrometry (MS) system in general includes an ion source for ionizing molecules of a sample of interest, followed by one or more ion processing devices providing various functions, followed by a mass analyzer for separating ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), followed by an ion detector at which the mass-sorted ions arrive. An MS analysis produces a mass spectrum, which is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios.
An ion guide is an example of an ion processing device that is often positioned in the process flow between the ion source and the mass analyzer. An ion guide may serve to transport ions through one or more pressure-reducing stages that successively lower the gas pressure down to the very low operating pressure (high vacuum) of the analyzer portion of the system. For this purpose, the ion guide includes multiple electrodes that receive power from a radio frequency (RF) power source. The ion guide electrodes are arranged so as to inscribe an interior (volume) that extends along a central axis from an ion entrance to an ion exit, and has a cross-section in the plane transverse to the axis. The ion guide electrodes are further arranged so as to generate an RF electric field that confines the excursions of the ions in radial directions (in the transverse plane). By this configuration, the ions are focused as an ion beam along the central axis of the ion guide and are transported through the ion guide with minimal loss of ions. This may be done in the presence of a gas flow so as to filter neutral gas species such as neutral atoms or molecules from the ion beam. An ion guide may also serve to transport ions through one or more stages wherein the gas pressure is maintained at a substantially constant level, such as in an ion mobility drift chamber or an ion collision cell.
The interior of an ion guide may be filled with a gas such that the ion guide operates at a relatively high (yet still sub-atmospheric) pressure. For example, a gas filled ion guide may be positioned just downstream of the ion source to collect the as-produced ions with as few ion losses as possible. Also, a buffer gas may be introduced into an ion guide under conditions intended to thermalize (reduce the kinetic energy of) the ions, or to fragment the ions by collision induced dissociation (CID). At relatively high levels of vacuum, the motions of ions are relatively easy to control. On the other hand, at elevated pressures collisions with gas molecules increasingly dominate the behavior of ion motion, making ion transmission at high efficiency more challenging. See Kelly et al., The ion funnel: Theory, implementations, and applications, Mass Spectrom. Rev., 29: 294-312 (2010).
An ion funnel is a type of ion guide in which the ion guide volume surrounded by the electrodes converges in the direction of the ion exit. In a typical configuration, the funnel electrodes are arranged as a series of rings coaxial with the ion guide axis. The ring-shaped electrodes are stacked along the ion guide axis and spaced from each other by small axial gaps. The inside diameters of the ring-shaped electrodes are successively reduced in the direction of the ion exit, thus defining the converging ion guide volume. The ion funnel can be useful for a number of reasons. The RF field applied by the converging geometry can compress the ion beam and increase the efficiency of ion transmission through the funnel exit. The large beam acceptance provided by the funnel entrance can improve ion capture, and the comparatively small beam emittance at the funnel exit can improve ion transfer into a succeeding device and can be closely matched to the size of the inlet of the succeeding device. The ion funnel can operate more effectively at higher pressures than a straight cylindrical ion guide. Thus, for instance, the ion funnel is useful for collecting ions emitted from an ion source without being impaired by large gas flows that may occur in the upstream region of the MS system. Also, as the ring electrodes are distributed in the axial direction and are able to be individually coupled to direct current (DC) circuitry, the ring electrodes can be directly utilized to generate a DC gradient along the ion guide axis to assist in keeping ions moving forward.
However, the effective potential (or “pseudo-potential”) of the RF field in ion funnels and other ion guides of stacked-ring geometry is non-zero on-axis (on the axis of symmetry). Instead, the effective potential forms a series of zeros or wells along the axis of symmetry. In practice, this is not much of a problem for higher-mass ions, but for low-mass ions these wells become problematic because they hinder the passage of the low-mass ions through the ion funnel. As a result, it is difficult to design an ion funnel that will work well for low-mass ions such as, for example, the lithium ion Li+ (m/z=7) commonly encountered in inductively coupled plasma-mass spectrometry (ICP-MS).
Therefore, it would be desirable to provide ion guides, including ion funnels, which address the problem of impaired ion travel caused by potential wells.